A hydraulically driven thick-material delivery pump, e.g., a two-cylinder pump for concrete, includes delivery cylinders are each driven by a hydraulic drive cylinder, that by whose alternating pressurization with the outlet pressure from a hydraulic pressure generator and depressurization of said drive cylinders to the tank of the hydraulic pressure generator a continuous concrete supply is achieved. In the event of a drastic load variation due to malfunctions occurring in the area of the thick-material pump, in order to prevent stalling of, e.g., a load-sensing main pump of the hydraulic pressure generator regulated to a constant flow volume, it is known to detect the rotational speed of the diesel engine electronically and when this speed drops beneath a threshold value considered critical, to control through an electrohydraulic control device, the delivery volume adjusting element of the main pump of the hydraulic pressure generator to reduce the volume flow generated by the main pump. This reduces the pump's torque requirement to the point that it can still be supplied by the drive motor and stalling of the latter can be avoided.
This type of overload protection of the diesel engine and of the hydraulic pressure generator as a whole is, however, overly complex from a technical standpoint and too trouble-prone for the rough operating conditions under which thick-material pumps must often be operated. The goal of the invention therefore is to provide a protective device of the type recited at the outset which, though simple in design, provides reliable overload protection for the hydraulic pressure generator, and in particular prevents its drive motor from stalling.
The overload protection device for the drive motor in accordance with the invention includes an auxiliary pump, flow resistance devices, and a pressure-controlled valve. The drive motor drives the main pump of the pressure generator, whose output flow rate is stabilized by a hydraulic regulating valve, which senses a pressure drop across a choke in the main hydraulic circuit connected to the main pump. The hydraulic regulating valve adjusts the output flow of the main pump such that when the pressure drop across the choke increases, the output flow rate of the main pump is reduced and when the pressure drop across the choke decreases, the output flow rate of the main pump is increased.
The auxiliary pump is also driven by the drive motor and generates an output flow rate proportional to the rotational speed of the drive motor. An auxiliary hydraulic circuit, including the first and second flow resistance devices connected in series, connects the auxiliary pump to the tank of the hydraulic pressure generator. The auxiliary hydraulic circuit includes a tapping point at which flow pressure in the auxiliary hydraulic circuit may be sensed. The pressure-controlled valve includes a control chamber connected with the tapping point of the auxiliary hydraulic circuit. The pressure-controlled valve is connected with the regulating valve of the hydraulic pressure generator and to the main consumer circuit connected to the main pump of the pressure generator. The pressure-controlled valve is biased by a pretensioned valve spring into a basic position that causes the second control chamber of the regulating valve to be relieved of the comparison pressure in the main consumer circuit. By charging the control chamber of the pressure-controlled valve with pressure prevailing at the tapping point of the auxiliary hydraulic circuit, the pressure-controlled valve may be moved into a functional position in which the comparison pressure prevailing in the main consumer circuit may be applied to the second control chamber of the regulating valve. With the overload protection device, if the rotational speed of the drive motor falls below a threshold value, the output flow generated by the auxiliary pump will not hold the pressure-controlled valve in a functional position. The pressure-controlled valve will instead move to its basic position, causing the regulating valve to be relieved of the comparison pressure from the main hydraulic circuit. The delivery volume of the main pump will as a result be reduced to a minimum delivery level, preventing the drive motor from being stalled.
The overload protection device operates reliably and can be provided at a relatively low cost.
In accordance with one embodiment of the invention, the first flow resistance of the auxiliary hydraulic circuit is connected to a pressure outlet of the auxiliary pump, and the second flow resistance is adjustable and is connected between the first flow resistance and the tank of the hydraulic pressure generator for sensing the flow rate of the auxiliary pump. A simple control valve designed for use at low control pressure levels will suffice for use as the pressure-controlled valve.
In accordance with another embodiment of the invention, the second flow resistance of the auxiliary hydraulic circuit is adjustable and connected to a pressure outlet of the auxiliary pump and the first flow resistance is positioned in the auxiliary hydraulic circuit between second flow resistance and the tank of the hydraulic pressure generator. Although a valve designed for high control pressure levels may be required for the pressure-controlled valve, it may be integrated in the auxiliary pump, and provide more sensitive control.
In accordance with another embodiment of the invention, the pressure-controlled valve may comprise a differential valve.
In accordance with another embodiment of the invention, the pressure-controlled valve may be designed as a proportional valve.
In accordance with another embodiment of the invention, the pressure-controlled valve comprises a 3/3-way valve, which, between the basic position, in which the regulating valve is cut off from the comparison pressure in the main consumer circuit and is connected instead with the tank of the hydraulic pressure generator, and the functional position, in which the regulating valve is connected with the comparison pressure in the main consumer circuit but is cut off from the tank of the hydraulic pressure generator, the pressure controlled valve also has a blocking functional position, in which the regulating valve is cut off from both the comparison pressure of main consumer circuit and the tank of the hydraulic pressure generator. The pressure-controlled valve may include a valve body with switching positions in which the pressure-controlled valve moves from the blocking functional position to the basic position or alternately to the functional position, such that when looking in the displacement direction of the valve body, the switching positions are arranged at an interval from one another equal to between 1/50 and 1/5, preferably 1/10, of the total stroke that the valve body can execute between end positions corresponding to the functional or basic positions. With this arrangement, oscillations of the outlet flow from the main pump may be avoided at low rotational speeds of the drive motor.
In accordance with another embodiment of the invention, the first flow resistance of the auxiliary hydraulic circuit comprises a rotational-speed-synchronous consumer, and the pressure drop across the second flow resistance of the auxiliary hydraulic circuit is between 5% and 15%, preferably 10%, of the pressure drop developing across the rotational-speed-synchronous consumer during steady-state operation of main consumer circuit. This arrangement has been found to increase the energy efficiency of the device.